The surface and bulk electrical properties of indium nitride
(InN) are determined by point and extended native defects to an
extent greater than in any other III-V semiconductor. The n-type
behavior of undoped InN thin films and the surface accumulation
layer of electrons reflecting surface Fermi level pinning high in
the conduction band are now well-established experimentally and
understood in terms of InN's large electron affinity, nearly 6
eV. However, the properties of Mg-doped material (is it really
p-type?) and the effect on electron concentration and transport
of the high density of extended defects was, until recently, less
well understood. A coordinated experimental approach using a
combination of electrical and electrothermal measurements will be
described that allows definitive evaluation of hole transport in
Mg-doped InN and, when combined with transport modeling based on
solutions to the Boltzmann transport equation, a quantitative
understanding of the crucial role of charged line defects in
limiting electron transport in this material. The use of
thermopower is especially noteworthy as it mitigates the effect
of the ubiquitous surface accumulation layer which had prevented
direct measurement of hole transport by Hall effect. The
extension of the present transport measurement methodology to
other systems including buried interfaces and heterojunctions
will also be described. [Preview Abstract]

Semiconducting alloys of GaN/ZnO are promising hosts for solar
photo-catalysis. The aqueous interface has been shown by Domen
and collaborators\footnote{K. Maeda {\it et al.}, Nature {\bf
440}, 295 (2006).} to catalyze water oxidation, a key half-
reaction in water splitting. We calculate by DFT, the
energetics of many (GaN)$_{1-x}$(ZnO)$_x$ supercell
configurations. Results show that significant short-range
order should be expected. A phase diagram is suggested from
free energies calculated versus temperature and concentration.
The mechanism of band gap bowing is examined. The DFT+U method
and hybrid functionals are used to reduce the problems of band
gap underestimation. Oscillator strengths near the band gap are
calculated, and an ensemble-averaged dielectric function is
constructed, with an aim to learn how to optimize solar light
absorption. [Preview Abstract]

It has long been known that GaN high-electron-mobility
transistors can
degrade significantly under hot electron stress. Meneghesso et al
[1]. showed
that GaN-based HEMTs are most prone to degradation in a state
with high
electric field and low current. More recently, an increase in the
yellow
luminescence was observed under similar stress conditions [2].
Hot electrons
can provide sufficient energy to cause a pre-existing defect to
convert into
a metastable configuration, release a hydrogen atom, or cause
migration of
pre-existing defects. We show that, among the possible known
defects in GaN,
the hydrogenated Ga vacancy has the properties that are needed to
account
for both the electrical degradation and the luminescence data.
\\[4pt]
[1] G. Meneghesso, et al., IEEE Trans. Dev. Mater. Reliab.
\textbf{8}, 332
(2008).
\\[0pt]
[2] C.-H. Lin, et al., Appl. Phys. Lett. \textbf{95}, 033510 (2009). [Preview Abstract]

GaN is widely used in the fabrication of High Electron Mobility Transistors
(HEMTs), but limited understanding of degradation mechanisms still hampers
applications. At moderate bias, an inverse piezoelectric effect in GaN
induces stress that often leads to cracking. A possible role of
stress-induced defects and defect migration remains elusive. Here we examine
the possibility that lattice stress may favor vacancy formation and
migration by strain relaxation as a possible precursor to crack formation
and device failure. We report results of first-principles density functional
calculations of the dependence of both Ga and N vacancy formation and
migration energies under strain and electric field and assess the impact of
these factors on degradation in GaN HEMTs. Migration barriers are calculated
by a Nudged Elastic Band (NEB) method. The results will be assessed against
experimental data. [Preview Abstract]

Although GaN devices have successfully entered technology, continued
development of nitride electronics is hampered by the limitations of p-type
doping. For this reason, we have employed electron paramagnetic resonance
(EPR) spectroscopy to study GaN:Mg grown with high Mg (1-4x10$^{20}$
cm$^{-3})$ and hole densities 1-40x10$^{18}$ cm$^{-3}$. EPR measurements are
made in the dark and under illumination at 4 K. Consistent with measurements
made on less heavily doped films, the Mg-related EPR signal exhibits a
sample-dependent anisotropy which depends on the hole density. Unlike lower
doped samples, however, the increased EPR signal intensity created by the
high hole density reveals photo-induced changes which suggest direct
defect-to-band transitions. Detailed stepped wavelength photo-EPR results
indicate that the Mg-related defect may be ionized with photon energy below
1.2 eV, likely related to capture of an electron from the valence band. A
second ionization near 2.3 eV remains to be understood. [Preview Abstract]

We studied InAlN/AlN/GaN heterostructures with In compositions
near 17{\%}
grown by Organo-Metallic Vapor Phase Epitaxy. These structures
are free of
strain and provide the confinement needed for a relatively high
density and
high mobility two-dimensional electron gas. The cross-sectional
TEM images
indicate the presence of an inadvertent GaN layer formed between
the InAlN
and AlN layers during the growth. Using Shubnikov-de Haas and Hall
measurements performed on gated Hall bar samples at 4.2 K, we
find that this
additional GaN layer acts as a parasitic conduction channel. The
quantitative mobility spectrum analysis of our data indicates
that this
parasitic channel has a very low mobility, and can be depleted by
the
application of a negative gate voltage. [Preview Abstract]

The pervasion of defects at the SiC:SiO$_{2}$ interface has limited the
performance and commercializing of SiC based transistors. While the defects
are believed to be related to excess carbon in the interfacial region, no
compelling microscopic models exists. Here we report the generation of
microscopic interfaces models for the SiC:SiO$_{2}$ interface. These models
include a 1 nm amorphous oxide and several layers of crystalline SiC. Defect
and defect reactions are explored. For instance, the 3-fold bonded carbon
defect is calculated to have an acceptor level at Ev + 1.4 eV close to the
value found experimentally and encouraging confidence in the methods
employed. The recently discovered beneficial effect of sodium ions motivates
our examination of the basic electrochemistry of the sodium ion interactions
with the ideal and defected interfaces. A comparison between microscopic
defect results and experiment will be presented. [Preview Abstract]

The application of SiC in electronic devices is currently
hindered by low carrier mobility at the the SiO$_2$/SiC
interfaces. Recently, it was found that thermally grown
SiO$_2$/4H-SiC interface can have a transition layer on the SiC
side with C/Si ratio as high as 1.2\footnote{T. Zheleva, et. al.
APL 93, 022108 (2008)} and the channel mobility of the
fabricated SiC MOSFETs decreases as the thickness of the
transition layer increases.\footnote{T. L. Biggerstaff, et. al.
APL 95, 032108 (2009)} However, the atomic structure of the
transition layer is unknown. Here we present the results of
first-principles calculations for silicon carbide with 20$\%$
excess carbon. Both static density functional theory calculations
and quantum molecular dynamics simulations were performed. We
reveal the structure that forms when a large amount of excess
carbon is incorporated into the lattice. In addition we explore
pairing and cluster formation. The overall results will be
assessed against experimental data. This work is supported in
part by NSF GOALI grant DMR-0907385. [Preview Abstract]

The channel mobilities in SiC-based metal-oxide-semiconductor
field-effect
transistors are significantly reduced by the interface defects
that produce
states in the band gap of the SiC-SiO$_{2}$ interface. Therefore,
it is of
great importance to investigate the nature of the interface
defects and the
ways for passivating such defects. We used first-principles
quantum-mechanical calculations to study the interface defects
due to
excessive carbon atoms. We report the results about the atomic
configurations of the defects and the associated electronic
structures, as
well as the effects of hydrogen and fluorine in passivating such
interface
defects. [Preview Abstract]

The electrical properties of planar 8H stacking-fault inclusions
(SFIs)
formed during epilayer growth on an 8 degree miscut n-type 4H-SiC
substrate
were characterized using nm-resolution BEEM. A $\sim $0.39 eV
lower Schottky
barrier was measured along the line where the SFI intersect a Pt
metal
overlayer, confirming that 8H SFIs are electron quantum wells (QWs).
Interestingly, an asymmetry of the BEEM current amplitude was
observed
around the intersection line, which is believed to be caused by
strong hot
electron reflection from the subsurface 8H inclusion. We will
discuss our
modeling to explain this strong hot electron reflection and the
lower bound
of hot electron attenuation length in 4H-SiC estimated from the
measured
BEEM current profile. Work supported by ONR. [Preview Abstract]

A recent study of Cu$_3$TaQ$_4$ (Q=S or Se) has shown that these
materials exhibit several favorable optoelectronic properties
including large optical band gap, tunable visible photoemission,
and p-type conductivity.\footnote{P.F. Newhouse et al, Thin Solid
Films {\bf 517}(2009) 2473} Cu$_3$TaQ$_4$ is unique among wide
band gap p-type semiconductors in that it crystallizes in a cubic
structure and is expected to show isotropic optical and
electronic properties. The origin of p-type conductivity in these
materials has been investigated using density functional theory
in the GGA approximation. The structure and energetics of point
defects have been determined using a supercell approach. We find
Cu vacancies to be the most likely origin of free hole carriers.
Compensation by donor like defects such as chalcogen vacancies is
estimated to be negligible because of high formation enthalpies.
Our study suggests that low overall defect concentrations are
achievable in Cu$_3$TaQ$_4$, raising the potential that these
materials could be used for p-type channel transparent transistors.
[Preview Abstract]

Bariumcopperchalcogenideflouride, BaCuQF (Q=S,Se,Te), is a family
of p-type wide band gap semiconductors with excellent
transparency over the whole visible part of the spectrum. Optical
properties can be continuously tuned in thin film solid solutions
of BaCuQF. Hole carrier concentration and mobility in BaCuQF
decreases from Q = Te$\rightarrow$Se$\rightarrow$S. Understanding
and control of the p-type conductivity in these materials is
crucial for potential applications in photovoltaics and
transparent electronics. We have investigated the origin of
p-type conductivity using ab initio density functional theory in
the GGA approximation. The structure and energetics of point
defects have been determined using a supercell approach. Cu
vacancies are the most likely origin of free hole carriers. Donor
like defects such as chalcogen vacancies lead to strong
compensation of charge carriers in BaCuSF and BaCuSeF but not
BaCuTeF. Hole concentrations obtained from a self-consistent
thermodynamic model reproduce the experimental trends. The
potential for charge carrier control through intentional doping
will be discussed.
[Preview Abstract]